Precipitated silicas find use in a broad range of manufactured products ranging from cosmetic and food products to industrial coatings to elastomeric materials, such as tires. Silicas are particularly useful in dentifrice products (such as toothpastes) where they function as abrasives and thickeners. Because of this functional versatility, and also because silicas, when compared to other dentifrice abrasives (notably alumina and calcium carbonate), have a relatively high compatibility with active ingredients like fluoride, there is a strong desire among toothpaste and dentifrice formulators to include them in their products.
However, it can be difficult to incorporate abrasive silicas into transparent dentifrice products. These transparent toothpaste products have become increasingly popular in recent years because of their greater appeal to some consumers and because they allow manufacturers to impart increased distinctiveness to their product. In order to produce a silica-containing transparent toothpaste, it is necessary that the silica's refractive index closely matches the refractive index of the toothpaste matrix, and that the silica has a high degree of light transmittance. Furthermore, in order to provide dental hygiene benefits, the silica must have sufficient abrasivity to provide cleaning of the tooth surfaces when incorporated into a dentifrice. Lastly, when incorporated in a transparent dentifrice, the silica should provide sufficient dentifrice viscosity build to make the transparent dentifrice convenient for consumer use.
Because the refractive index of the silica must match the refractive index of the toothpaste matrix in order for the toothpaste to be transparent, typically the concentration of water in the toothpaste must be maintained at relatively low levels. Water generally has a far lower refractive index than silica, glycerin and sorbitol: commercially available precipitated silicas have a refractive index of about 1.438 to 1.451, while water has a refractive index of 1.332, 98% glycerin has a refractive index of 1.472 and 70% sorbitol has a refractive index of 1.456. As the toothpaste's water concentration increases, the refractive index of the toothpaste decreases, and thus, in order for the refractive index of the silica to match the refractive index of the toothpaste, the water concentration in the toothpaste must be minimized. This is undesirable because water is generally the least expensive toothpaste component, and decreases in water concentration are normally offset by increases in humectant concentration (which is quite expensive). Thus, decreasing water concentration will cause a corresponding increase in the toothpaste unit cost.
Furthermore, an abrasive silica is an indispensable ingredient in a transparent toothpaste for providing effective dental cleaning performance. Unfortunately adding an abrasive silica can reduce the transparency of the overall toothpaste product because of its low degree of transmittance and high refractive index. Because of the silica's high refractive index, it is often necessary to reduce the water concentration while increasing the humectant concentration, which results in a significant increase in product cost.
Another consideration for producing a transparent toothpaste is related to the toothpaste viscosity. Most commercial toothpastes have a viscosity range of between 250,000 cps to 1,000,000 cps. When the viscosity is less than 250,000 cps, the toothpaste is very thin and has poor stand-up characteristics, so that the toothpaste sinks into the bristles of the toothbrush and drips from the brush. When the viscosity is greater than 1,000,000 cps, the toothpaste becomes very difficult to squeeze from the tube and less likely to have good dispersion in the mouth.
Typically, the viscosity build of a toothpaste is controlled through the use of silica, or gelling agents, such as polysaccharides or carboxymethyl cellulose. The gelling agent is usually present in low concentrations of about 0.1 to 1.5 wt % of the toothpaste composition, because higher concentrations of gelling agents can cause problems with product dispersion, rheology, and lumping. Because the gelling agent can only be used in these low concentrations, most toothpaste formulations are dependent on the silica component to increase the viscosity build of the toothpaste to a satisfactory level. But if a silica with low structure and low oil absorption is used, then high loading levels of silica are required to build the toothpaste to the required viscosity. By contrast, very high structure silica provides good viscosity build, but does not provide adequate abrasiveness for tooth cleaning.
Given the foregoing, there is a continuing need for a silica composition that not only provides excellent abrasive performance and high oil absorption (allowing for good viscosity build), but also has good optical properties such as a relatively high degree of transmittance, and an index of refraction that is sufficiently low, such that the silica can be included in a transparent toothpaste composition having a relatively high concentration of water.